hydrogen ion

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The positively charged ion of hydrogen, H⁺, formed by removal of the electron from atomic hydrogen and found in the form of hydronium ion in all aqueous solutions of acids.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun chemistry Used other than as an idiom: see hydrogen,‎ ion. ; H- or H+
• noun chemistry a proton combined with one or more water molecules; usually written H3O+ and called the hydronium ion though is best considered as H9O4+ but is often written H+(aq) for simplicity
• noun chemistry a bare hydrogen nucleus; a proton , deuteron or triton

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a positively charged atom of hydrogen; that is to say, a normal hydrogen atomic nucleus